Question: Which of these numbers is prime? ${3,\ 25,\ 56,\ 63,\ 99}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 3 are 1 and 3. The factors of 25 are 1, 5, and 25. The factors of 56 are 1, 2, 4, 7, 8, 14, 28, and 56. The factors of 63 are 1, 3, 7, 9, 21, and 63. The factors of 99 are 1, 3, 9, 11, 33, and 99. Thus, 3 is a prime number.